Chemical and thermal evolution of the late Cretaceous Tuolumne intrusive suite, Yosemite National Park, California.

摘要

Thermal and chemical models were developed to investigate the emplacement and evolution of the Tuolumne intrusive suite (TIS), a large nested plutonic complex located in Yosemite National Park, California. Previous U-Pb zircon ages show that the TIS was magmatically active for approximately 10 My, from approximately 94.5 Ma to 84.5 Ma. Ages decrease systematically from the margins inwards. In general, the age distribution is inconsistent with emplacement as a large single magma chamber. Assuming the entire TIS was initially molten, thermal modeling predicts complete solidification in less than 1 My, a much shorter time than suggested by the U-Pb zircon ages. Instead the geochronology and thermal modeling results suggest incremental emplacement from a series of sheets or dikes. New U-Pb data generated in this study suggests that emplacement of the Half Dome granodiorite was broadly continuous, consistent with the dike emplacement hypothesis.; Major-element data display linear trends with SiO2, but the trace elements show considerable scatter. Correlations between incompatible trace elements are generally absent. In contrast to the major and trace elements, initial Sr and Nd isotopic ratios are well correlated with distance across the pluton, varying from approximately Sri = 0.7057 and epsilon Nd(t) = -3 at the margins, to Sri = 0.7064 and epsilon Nd(t) = -8 at the center. These results are inconsistent with closed-system fractional crystallization. In fact, removal of any combination of major minerals cannot explain the observed major- and trace-element trends. However, a model which assumes random mixing between partial melts of isotopically less evolved mafic to mantle-like source rocks, and isotopically more evolved crustal-like source rocks predicts isotopic, major- and trace-element trends consistent with the data. Incremental addition of these melts to the growing pluton can account for the observed lack of geochemical and spatial coherence. Aluminum-in-hornblende barometry suggests that final solidification of the TIS occurred at a depth of approximately 6 km.